Thursday, May 07, 2009

Gasifying Algae to Synthetic Natural Gas: Treating Algae Like Any Other Biomass

Most algae-to-fuels projects aim to extract large quantities of oil from algae for biodiesel production. But algae is even more prolific at producing biomass than it is at producing oil. A biomass gasification process developed by the Pacific Northwest National Lab is able to quickly turn algae into synthetic natural gas (SNL) -- 400 times faster than anaerobic digestion!

More than 99% of the biomass is gasified to yield both a product gas and steam, which contains the carbon dioxide produced during gasification. After condensation, the water enriched with dissolved carbon dioxide is recycled to the growth ponds to accelerate growth of the next generation of biomass while reducing emissions to nearly zero.

The PNNL gasifier runs at relatively low temperatures—350 °C compared with 700 °C or more for other systems—in a small stainless steel reactor. Compared with other methods of gasifying biomass, such as anaerobic digestion, PNNL’s process works 400 times faster and gives higher yields.

According to Doug Elliott, the PNNL scientist who invented the gasification process, “It is simple—we put wet biomass like algae in the gasifier, where it is catalytically converted, and we collect fuel gas and byproducts. It’s serendipity that our system creates carbon dioxide as a byproduct that Genifuel needs naturally to grow the algae. It’s a completely green process.”

The technology behind the gasification process has been under development for a number of years. PNNL scientists have achieved significant advances in the chemistry of catalysts and the selection of the optimum temperatures and pressures for the process, as well as improving the systems to protect the catalyst from impurities in the biomass.

Genifuel grows aquatic biomass, such as algae, in shallow ponds or troughs, then harvests and processes the biomass for conversion using the PNNL technology. Water used in the growth ponds doesn’t have to be high-quality fresh water, and can be treated wastewater, brackish or alkaline water, or even salt water, Oyler said. Non-crop land can be used, so the process doesn’t compete with food production. _GCC

Although this process yields gaseous fuel rather than liquid, it could easily be modified to produce a wide range of high-value chemicals from algae biomass.

Another important point: Even most algae to biodiesel processes need to do something with the biomass residue. This gasification process may very well fit into more broad-spectrum algae production facilities to provide a wider range of production.